AM8121 24dB VCF (100M)

Introduction This module is a clone of the Voltage Controlled Filter from the legendary Roland 100M analog modular synthesizer which is a simple non-voltage controlled single pole 6dB High Pass Filter in front of a fully voltage controlled 4-pole 24dB Low Pass Filter with resonance into full self oscillation. This is the “classic” Roland 4-pole OTA based Low Pass Filter, first developed for the Jupiter 4 and SH-7 in 1978 using discrete components.

My version makes use of the BA6110 OTA chip and produces a nice warm sound with a well controlled resonance. The Roland OTS filter design was gradually miniaturized, first as the  IR3109  VCF chip in the Jupiter 8 (see the AM8109 module) and later as the D80017 VCF and VCA chip in the Juno 106 (see the AM8106).

Detailed Description The AMSynths design is a careful replication of the original Roland 100M design with the HP and LP filters in series.  The circuit has been modified to make use of easily available 1K Tempco and to use USA transistors (2N3904/06). We recommend 1% polystyrene capacitors for the filter stages and upgrades to the audio path capacitors as well. The Op Amp used in the High Pass filter has been upgraded from a 4558 to a TL071, and the 2SK30 FET transistors after each OTA filter pole have been replaced with easy to locate 2N5485 FET.

I have not utilised the BA6110 buffer and 2N5485Jfets have been used instead. The feedback circuit that provides resonance and self oscillation has two diodes which act as voltage limiters preventing the feedback running away and ever increasing self oscillation. The filter was redesigned in 2017 to be skiff rack friendly and it retains the easier to source USA transistors.

Module Availability I built a couple of these modules in 2009 and in 2018 developed slider pot versions as commercial products. However in 2020 with the arrival of Behringer clones of the 100M modules, I cancelled the project. However in late 20022 I started developing a new range of 100M module in Eurorack, including a dual SVF filter – the AM8122. It is unlikely I will recreate the AM8121 as the Behringer module is pretty close to the original and self oscillates down to 20Hz.

Behringer 121 Modification The only limitation is that the 121 uses SMD ceramic filter capacitors which should be replaced with polypro or polystyrene versions to provide a better quality sound.

Copyright AMSynths 2022